Superconductor materials have long been known and understood by the technical community. Low-temperature superconductors (low-tc or lts) exhibiting superconducting properties at temperatures requiring use of liquid helium (4.2 k), have been known since 1911. However, it was not until somewhat recently that oxide-based high-temperature (high-tc) superconductors have been discovered. Around 1986, a first high-temperature superconductor (HTS), having superconducting properties at a temperature above that of liquid nitrogen (77K) was discovered, namely yba2cu3o7−x (YBCO), followed by development of additional materials over the past 15 years including bi2sr2ca2cu3o10+y (BSCCO), and others. The development of high-tc superconductors has created the potential of economically feasible development of superconductor components and other devices incorporating such materials, due partly to the cost of operating such superconductors with liquid nitrogen rather than the comparatively more expensive cryogenic infrastructure based on liquid helium.
Of the myriad of potential applications, the industry has sought to develop use of such materials in the power industry, including applications for power generation, transmission, distribution, and storage. In this regard, it is estimated that the inherent resistance of copper-based commercial power components is responsible for billions of dollars per year in losses of electricity, and accordingly, the power industry stands to gain based upon utilization of high-temperature superconductors in power components such as transmission and distribution power cables, generators, transformers, and fault current interrupters/limiters. In addition, other benefits of high-temperature superconductors in the power industry include a factor of 3-10 increase of power-handling capacity, significant reduction in the size (i.e., footprint) and weight of electric power equipment, reduced environmental impact, greater safety, and increased capacity over conventional technology. While such potential benefits of high-temperature superconductors remain quite compelling, numerous technical challenges continue to exist in the production and commercialization of high-temperature superconductors on a large scale.
Among the challenges associated with the commercialization of high-temperature superconductors, many exist around the fabrication of a superconducting tape segment that can be utilized for formation of various power components. A first generation of superconducting tape segment includes use of the above-mentioned BSCCO high-temperature superconductor. This material is generally provided in the form of discrete filaments, which are embedded in a matrix of noble metal, typically silver. Although such conductors may be made in extended lengths needed for implementation into the power industry (such as on the order of a kilometer), due to materials and manufacturing costs, such tapes do not represent a widespread commercially feasible product.
Accordingly, a great deal of interest has been generated in the so-called second-generation HTS tapes that have superior commercial viability. These tapes typically rely on a layered structure, generally including a flexible substrate that provides mechanical support, at least one buffer layer overlying the substrate, the buffer layer optionally containing multiple films, an HTS layer overlying the buffer film, and an optional capping layer overlying the superconductor layer, and/or an optional electrical stabilizer layer overlying the capping layer or around the entire structure. However, to date, numerous engineering and manufacturing challenges remain prior to full commercialization of such second generation-tapes and devices incorporating such tapes.
Significantly, the critical current of the HTS tape can be strongly affected by the presence of strong magnetic fields. Further, the angle of the magnetic field to the tape significantly affects the critical current. For example, depending on the angle of the magnetic field, the critical current can be reduced by a factor of seven to ten at 1-tesla (t) and 77K compared to the critical current in the absence of a magnetic field. One particular challenge is to reduce the effect of magnetic fields on the critical current of the HTS tape. Additionally, the angular dependence of critical current in the presence of a magnetic field shows a significant anisotropy with a peak in critical current when the field is orientation parallel to the tape and a sharp reduction in critical current as the field is moved away from this orientation. Therefore, another challenge is to improve the critical current in field orientations other than that parallel to the tape. Thus, there remains a need for HTS tapes that have improved performance in strong magnetic fields.